The Two-Mile Time Machine by Alley Richard B

Author:Alley, Richard B.
Language: eng
Format: epub
Publisher: Princeton University Press
Published: 2000-11-20T16:00:00+00:00

Yo-Yos

By now, it may be getting hard to keep score, so let’s recap. During the last million years, ice grew for about 90,000 years as we sank into the global cold of an ice age, with smaller wiggles spaced about 19,000, 23,000, and 41,000 years apart. Ice then shrank for 10,000 years as we warmed into an interglacial period. This pattern repeated, over and over, in response to changing sunshine linked to features of Earth’s orbit. The climate was most nearly stable (although still quite variable) during the coldest and warmest times. But during the coolings into the ice ages and warmings from ice ages over the last 100,000 years (and, as we shall see, during much of at least the last million years), the climate in the north Atlantic jumped between several cold centuries and several warm ones. Through a few of these Dansgaard-Oeschger cycles, each warm event was a little cooler than the one before. Then the ice sheet in Hudson Bay suddenly dumped a great mass of icebergs into the north Atlantic in a Heinrich event. The next warming was especially large, and then the Bond cycle of progressively cooler events resumed. You might think of a roller coaster riding the orbital rails, with Heinrich-Bond bungee-jumping off the roller coaster while playing with a Dansgaard-Oeschger yo-yo.

The most recent orbital cycle reached the coldest part of the ice age about 20,000 years ago, followed by a 10,000-year warming trend to reach our current mild, stable climate. The warming trend was punctuated by several abrupt warmings and coolings. The climate cooled into the Younger Dryas—the youngest of the large changes—about 12,800 years ago, and warmed again about 11,500 years ago. Because the Younger Dryas occurred after the world had begun warming from the depths of the most recent ice age, the icebergs of Younger Dryas time melted faster and did not carry rocks as far as some older icebergs did. But in many places close to Hudson Bay, the Younger Dryas looks like a seventh Heinrich event.

Much effort has gone into tracking the footprint of the Younger Dryas around the globe, with considerable success as described above. The older Dansgaard-Oeschger and Heinrich-Bond signals have been harder to follow. Many records of the Younger Dryas have come from lakes and bogs that were hollowed out by glaciers that began melting only a little before the Younger Dryas, so these lakes and bogs don’t have older records. Other records become harder to date as they get older, so correlations are harder to achieve. Finally, however, the picture is beginning to come clear.

The available data now indicate that the cold times of both Dansgaard-Oeschger and Heinrich-Bond oscillations in Greenland and around the north Atlantic also were cold, dry, and windy in broad regions extending into subtropical Africa and Asia, and across Europe and North America. Much of the world probably became drier because the cooling reduced water vapor, although certain regions became especially dry as storm tracks shifted away from them to areas that became somewhat wetter.

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